Correctly handle `multipart/form-data` rather than `application/octet-stream`

[DavidGarfinkle]

currently, just a piece ID and the symbolic data is stored, but no metadata

[DavidGarfinkle]

add author and piece name

[DavidGarfinkle]

i attempted to implement this correctly by handling a Content-Type: multipart/form-data MIMETYPE as specified by RFC 2388 and implemented in requests and flask

after two hours I gave up because the werkzeug.Request.files and werkzeug.Request.form attributes were always empty, similar but not identical to the following github issues:

• request.files is empty after POSTing a file psf/requests#2505
• request.files is empty after POSTing a file pallets/flask#1384
• Posting files with unicode file names psf/requests#2883

Instead, I implemented this by handling ONLY mimetype application/octet-stream (binary data) and requiring the following binary payload:

1. JSON metadata encoded in UTF8
2. hardcoded random 16-byte separator value 90dc2e88fb6b4777432355a4bc7348fd17872e78905a7ec6626fe7b0f10a2e5a
3. Musical data to be parsed by music21